The present invention relates to a vehicle brake system comprising an electrical control logic, an electrically controllable pressure modulator associated with each brake-actuating device, and a brake-signal transmitter having an electrical output signal which is supplied to the control logic.
A vehicle brake system of the aforementioned type is known from German Patent DE-OS No. 2,128,169. The disclosed system includes a control logic the output signals of which control an electric motor of the pressure control unit. The pressure control unit comprises a hydraulic pressure modulator including a piston actuated by the electric motor and having a wheel brake cylinder applying the brake shoes connected to it. The pressure is monitored by a hydraulic-mechanical final control element which influences a first variable ohmic resistor. The brake-signal transmitter includes a second mechanically variable ohmic resistor. The brake pedal which is actuatable in opposition to a spring is mechanically connected with the sliding contact of the second variable ohmic resistor. The control logic includes a Wheatstone bridge in which each variable resistor interconnects with a fixed resistor to form branches which are paralleled to a power source.
The output signal of the control logic is tapped between the points of the variable and fixed resistors. In the inactive state, the final control element signals no pressure, and the brake-signal transmitter issues no signal. The Wheatstone bridge is in a balanced state.
When the brake pedal is depressed resulting in the generation of a brake signal, a difference of potential will occur at the Wheatstone bridge and the electric motor serving to modulate the pressure will be actuated. With the pressure increasing, the final control element will change the first ohmic resistor until it is matched to the first fixed ohmic resistor so that the previously unbalanced Wheatstone bridge is again balanced. Then the control logic ceases to issue an output signal, and the pressure build-up in the wheel brake cylinders will be terminated and will remain constant. In such a vehicle brake system, the pressure in the wheel brake cylinders is directly determined by the position of the vehicle's brake pedal, like in a conventional vehicle hydraulic brake system. Thus, each position of the brake pedal corresponds to a specific brake pressure.
It is a disadvantage in this arrangement that the resistors of the Wheatstone bridge are located in different temperature environments. Thus, each variable resistor allocated to the pressure control units is in an environment which, due to operation, is subject to high variations in temperature. In contrast, the variable resistor of the brake-signal transmitter is located in the passenger compartment where it is exposed to a temperature of about 19.degree. C. The control logic is arranged in either the passenger or engine compartment. High differences in temperature as they may occur under adverse operating conditions would thus be the cause of trouble and error signals would be generated by the control device. In addition, false braking and the build-up of different pressures in the individual wheel brake cylinders are conducive to an increased stopping distance and directional instability during braking. Hence, the output signal generated by the brake-signal transmitter depends on the pressure in the wheel brake cylinders, on the one hand, and on the temperature in the first and second variable resistor environments, on the other hand.